Method of detecting an optical transmission line

ABSTRACT

A method of detecting an underground optical transmission line effectively and efficiently regardless if the optical transmission line is of a non-metallic type or not. The method comprises steps of emitting acoustic vibration from vibration generating means toward an underground area where the optical transmission line 1 is laid while moving the vibration generating means on a road roller 5 on the ground and measuring changes in the intensity of an optical signal being transmitted through the optical transmission line 1 subjected to the vibration and the displacement of the vibration generating means to determine the location of the underground optical transmission line 1 from the distribution of the intensity of the transmitted optical signal.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to a method of detecting a particular undergroundoptical transmission line from the surface of the ground.

2. Prior Art

In modern telecommunication systems, optical cables are laid along theroute of installation directly or housed in a duct or conduit which islying underground.

Typical underground optical telecommunication networks comprise centralstations, relay stations and terminal stations interconnected by meansof a large number of optical transmission lines, which are oftenramified, looped and star-connected.

There often arises cases where a particular underground opticaltransmission line needs to be branched, moved, tested, repaired andreplaced.

Then, the particular underground optical transmission line has to bedetected and dug out from the surface.

Since the routes of underground optical transmission lines aredocumented, it is normally not difficult to roughly trace the route of aparticular transmission line from the ground surface and select a sitefor digging out the line. However, tremendous efforts may be required toprecisely locate a particular transmission line at the work site simplybecause it is not visible from the surface. An old and conventionalmethod to alleviate such efforts is to put a sign, or indicator, on theground, telling that one or more than one optical transmission lines areburied there.

As more and more underground optical transmission lines are laid to formlarge networks, however, installation of such signs will become costlyand, in some cases, may constitute obstacles for traffic on the ground.

Additionally, such indicators will eventually become damaged, degradedand inoperative as they are exposed to the atmosphere for a prolongedperiod of time.

In short, the use of signs does not provide an effective way fordetecting underground optical transmission lines and is accompanied by aproblem of necessity of digging the ground over a large area to take outa single transmission line. If an excavator machine is used toefficiently dig the ground over a large area, it can accidentallydestroy transmission lines and/or other underground installations.

It should be noted here that an underground optical transmission linecomprising an electric conductor or a metallic component can beprecisely located by sending an electric signal into the conductor anddetecting from the ground surface the electromagnetic wave generated bythe conductor according to the electric signal.

As this technique is effective only for optical transmission linescomprising a conductor, the use of the technique will inevitably be verylimited under the current circumstances where non-metallic opticaltransmission lines are more and more prevalent in opticaltelecommunication systems.

SUMMARY OF THE INVENTION

In view of the above discussed technological problems, it is thereforean object of the present invention to provide a method of accurately andefficiently detecting a underground optical transmission line regardlessif it is a dielectric type or not.

According to the invention, the above object is achieved by providing amethod of detecting an underground optical transmission line comprisingsteps of emitting acoustic vibration from vibration generating meanstoward an underground area where the optical transmission line is laidwhile moving the vibration generating means on the ground and measuringchanges in the intensity of an optical signal being transmitted throughthe optical transmission line subjected to the vibration and thedisplacement of the vibration generating means to determine the locationof the underground optical transmission line from the distribution ofthe intensity of the transmitted optical signal.

The method of detecting an optical transmission line according to theinvention can be used to detect not only a particular opticaltransmission line contained in an optical cable but also an optical lineburied underground integrally with or in parallel with anotherunderground utility installation such as a water supply pipe, a sewerpipe or a gas supply pipe.

According to an aspect of the present invention, an optical transmissionline is searched two-dimensionally in the first step by using any giventwo different horizontal directions to determine a spot from which theground is to be dug and thereafter it is vertically searched todetermine the depth to which the ground is dug to reach the line.

According to a particularly advantageous aspect of the presentinvention, the state of polarization of the signal running through theoptical transmission line to be detected is modulated to change theintensity of the output signal or tile optical coupling coefficient ismodulated at the output terminal of tile optical transmission line tochange the intensity of the output signal of the transmission line.

According to another aspect of the invention, a single mode type opticalfiber comprised in the optical transmission line to be detected canadvantageously be used for tile detection.

According to still another aspect of the invention, a multi-mode typeoptical fiber comprised in the optical transmission line to be detectedcan advantageously be used for the detection when the multi-mode typeoptical fiber is connected to a single mode type optical fiber at theoutput terminal thereof.

When such is the case, the speckle pattern produced at the connection ofthe multi-mode type optical fiber and the single mode type optical fiberis varied to change tile intensity of the output signal of themulti-mode type optical fiber of the optical transmission line.

When a single mode type optical fiber comprised in an opticaltransmission line to be detected is used for the detection, a techniqueof light wave control utilizing acoustic optical phenomena (ultrasonicpolarization or modulation) can be employed such that a vibration signalis applied to the signal light being transmitted along the single modetype optical fiber from the outer peripheral surface of the opticalfiber to translate any changes in the signal light into correspondingchanges in the polarization of the light.

Then, the signal light is caused to have a frequency same as that of theapplied vibration signal and an intensity proportional to that of theapplied vibration signal.

If vibration is emitted toward the underground optical transmission lineto be detected from the surface of the ground for light wave control,although it attenuates as it is transmitted through the ground, theintensity of the vibration signal applied to the signal light in asingle mode type optical fiber of the optical transmission line and,therefore, that of the output signal light of the single mode typeoptical fiber whose state of polarization is modulated is also maximizedwhen the source of vibration approaches closest to (and right above) theoptical transmission line.

Thus, according to the method of the present invention, a particularunderground optical transmission line can be detected simply by emittinga vibration signal from moving vibration generating means toward theunderground optical transmission line to be detected in order to findout a spot where the intensity of the signal light being transmittedthrough a single mode type optical fiber of the optical transmissionline is maximized. Then, the optical transmission line is located rightbelow the spot of the vibration generating means.

When a multi-mode type optical fiber comprised in an opticaltransmission line to be detected is used for the detection, the outputterminal of the fiber is connected to a single mode type optical fiberand then vibration is applied to the multi-mode type optical fiber sothat interference of light among the different modes of the multi-modetype optical fiber caused by the vibration applied to the multi-modetype optical fiber may be utilized for the detection.

As vibration is applied to the multi-mode type optical fiber under suchconditions, there arises interference of signal light among thedifferent modes of the multi-mode type optical fiber that aretransmitting the signal light at the connection of the multi-mode typeoptical fiber and the single mode type optical fiber to produce aspeckle pattern on the cross section of the core of the multi-mode typeoptical fiber, indicating that, the intensity of light is not evenlydistributed in the multi-mode type optical fiber.

While the pattern of interference of light may be changed by externaldisturbances other than the vibration applied to the multi-mode typeoptical fiber, it may most conspicuously show changes when a vibrationsignal is applied thereto from moving vibration generating means on theground.

When such changes are observed at a point on the core of the multi-modetype optical fiber, the intensity of the signal light will vary as afunction of the frequency of the applied vibration signal.

Additionally, such changes can be expressed by a signal representingchanges in the intensity of light that occur in the single mode typeoptical fiber connected to the multi-mode type optical fiber as afunction of the frequency of the applied vibration signal.

Thus, by translating the signal representing changes in the intensity oflight into a signal representing modulation of the state of polarizationof the light signal being transmitted through the optical fiber, anoptical transmission line comprising single mode type optical fibers canbe detected as easily as an optical transmission line comprising singlemode type optical fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a plan view of a situation where the method of the presentinvention is used to detect an underground optical transmission line.

FIG. 1(b) is a sectional view cut along X--X line of FIG. 1(a).

FIG. 2 is a block diagram of an optical detection system to be used forthe method of the present invention.

FIG. 3 is a graph showing the waveform of an output signal modulated byapplying vibration according to the method of the present invention.

FIG. 4 is a graph showing the relationship between the distance from asource of vibration to an optical transmission line and the intensity ofthe output signal light of the transmission line modulated by thevibration.

FIG. 5 is a block diagram similar to FIG. 2 but showing theconfiguration of an alternative optical detection system to be used forthe method of the present invention.

BEST MODES OF CARRYING OUT THE INVENTION

FIGS. 1(a) and 1(b) illustrate a typical situation where the method ofthe present invention is used to detect an underground opticaltransmission line.

FIGS. 1(a) and 1(b), the optical transmission line 1 is in fact a doublecore SZ-type optical cable comprising a pair of single mode type opticalfibers 2.

The optical transmission line 1 is approximately 100 m long and is laidunderground in a bare state at a depth of 750 mm from the surface.

As shown in FIG. 1(a) , the single mode type optical fibers 2 of theoptical transmission line 1 are drawn out of the optical transmissionline 1 at both ends.

More specifically the left ends of the single mode type optical fibers 2in FIG. 1(a) are interconnected to from a loop, while the right oropposite ends of the single mode type optical fibers 2 are connected toa measuring instrument 4.

Thus, when extended, the total length of the single mode type opticalfibers 1 will be approximately 250 m including the portions thereofdrawn out of the optical transmission line 1.

As shown in FIG. 4, the measuring apparatus 4 comprises a light source6, an analyzer 7, a photodetector 8, a O/E converter 9 and anoscilloscope.

An LD of any known type having a high degree of coherency is used forthe light source 6 to shed light into each of the single mode typeoptical fibers 2 of the optical transmission line 1. The analyzer 7, thephotodetector 8, the light/electricity transducer 9, the oscilloscope 10may be of any known types.

The analyzer 7 is so adjusted as to maximize the intensity of signallight having a changing state of polarization. The level of the lightemitted from the light source is -0.7 dBm at the point of emission,while that of the light at the point of reception of the photodetector 8is approximately -0.62 dBm.

The oscilloscope 10 observes the output of the light/electricitytransducer 9 and records the result of observation.

In FIG. 2, there is provided a road roller 5 that carries on it avibration generating apparatus (not shown) for transmitting vibrationtoward underground.

The vibration generating apparatus comprises a source of ultrasonicvibration or any other form of mechanical vibration.

In an experiment conducted by the inventor of the present invention in asituation as illustrated in FIGS. 1(a) and 1(b) where an opticaltransmission line 1 is laid underground, a road roller 5 carrying avibration generating apparatus was made to run on the ground above andnear the route of the optical transmission line 1 and the vibrationgenerating apparatus on it was caused to transmit vibration having afrequency of approximately 55 Hz.

The optical transmission line 1 comprised a pair of single mode typeoptical fibers 2, through each of which signal light was beingtransmitted.

The output signal light of the photodetector 8 was then converted intoan electric signal by the O/E converter 9 to obtain a waveform having afrequency same as that of the applied vibration on the screen of theoscilloscope 10 as illustrated in FIG. 3.

The intensity of the modulated output signal does not represents anabsolute value.

More specifically, the intensity of the output signal is a relativevalue expressed as a function of various conditions of signaltransmission, where the relationship between the distance from thesource of vibration to the optical transmission line 1 and the amplitudeof the intensity, or the voltage, of the output signal is illustrated bythe graph of FIG. 4.

It is obvious from FIG. 4 that the vibration emitted toward theunderground optical transmission line 1 from the surface of the groundwas very effective for detecting the transmission line.

In other words, the vibration emitted downward from a spot on the groundsurface right above the optical transmission line 1 laid underground ina bare state remarkably amplified the intensity of the signal lightrunning through the transmission line and having a changing state ofpolarization and consequently the optical transmission line 1 (object ofdetection) could be easily and accurately detected.

So, it will be clear by now that, when the signal light running throughthe underground optical transmission line 1 was subjected to vibrationemitted from the surface of the ground, it sensitively responded to thevibration to change the state of polarization and so an opticaltransmission line 1 can be detected by utilizing this effect of acousticvibration on signal light running through an underground opticaltransmission line that becomes apparent when it, is applied to theoptical transmission line from the surface of the ground.

FIG. 5 shows a block diagram similar to FIG. 2 but showing theconfiguration of an alternative optical detection system to be used forthe method of the present invention.

This optical detection system can be used for an optical transmissionline 1 comprising multi-mode optical fibers (GI) 22, each of which isconnected at an end to a single mode type optical fiber (SM) 20 by anconnector section 21 disposed between the GI optical fiber and the SMoptical fiber.

With an arrangement as described above, the speckle pattern that appearsat the connector section 21 changes between the multi-mode type opticalfiber and the single mode type optical fiber to change the intensity ofthe signal light sent out from the multi-mode type optical fiber. So theoptical transmission line 1 can be located by detecting the changes inthe intensity of the signal light.

An underground optical transmission line 1 can be three-dimensionallyexpressed in-terms of longitudinal axis (y-axis), transversal axis(x-axis) and vertical axis (z-axis) of a rectangular coordinate system.

In view of such a coordinate system, an optical transmission line 1 islocated only two-dimensionally (in terms of x- and y-axes) by any of theabove described optical detection systems.

The method of the present invention, however, can be used to locate anoptical transmission line 1 not only two-dimensionally but also in termsof the depth (z-axis) at which the optical transmission line 1 laidunderground in a roamer similar to the one as described above.

Assuming that an underground optical transmission line 1 has beenlocated two-dimensionally in terms of x- and y-axes and now an excavatoris used to dig out the optical transmission line 1. Then an opticaldetection system to be used with the method of the present invention isfitted to or disposed close to the excavator so that vibration can beemitted from the vibration generating apparatus of the system toward theoptical transmission line 1, while the excavator is digging the groundnear the optical transmission line 1.

As the excavator digs the ground deeper, the optical detection systemcomes close to the optical transmission line 1 and the intensity of thesignal light being transmitted from the optical transmission line andreceived by the optical detection system will be grow accordingly.

More specifically, as the intensity of the signal light received by theoptical detection system is increased inversely proportionally to thedistance between the optical transmission line 1 and the excavator and,therefore, the optical detection system disposed on or near theexcavator, the depth at which the optical transmission line 1 is laidunderground can be determined by observing the varying intensity of thesignal light as a function of the distance from the optical detectionsystem or the excavator.

Thus, if an optical detection system as described above is used with anexcavator digging the ground to expose an optical transmission line, theoptical transmission line 1 can be dug out without being damaged by theexcavator.

The method of detecting an optical transmission line according to theinvention can be used for locating an optical transmission line in theform of one or more than one optical fibers, optical cords or opticalcables buried underground integrally with or in parallel with anotherunderground utility installation such as a water supply pipe, a sewerpipe or a gas supply pipe made of synthetic resin (FRP), ceramic orconcrete in a manner as described above.

INDUSTRIAL APPLICABILITY

As described above in detail, with the method of detecting an opticaltransmission line according to the invention, vibration generating meansemits acoustic vibration toward an underground area where the opticaltransmission line is laid while the vibration generating means is movedon the ground and changes in the intensity of an optical signal beingtransmitted through the optical transmission line and subjected to thevibration are measured to determine the location of the undergroundoptical transmission line from the distribution of the intensity of thetransmitted optical signal as a function of the displacement of thevibration generating means. Thus, the underground optical transmissionline can be effectively and efficiently detected regardless if theoptical transmission line is of a non-metallic type or not.

What is claimed is:
 1. A method of detecting an underground opticaltransmission line, comprising emitting acoustic vibration from avibration generating means toward an underground area where the opticaltransmission line is buried, moving the vibration generating means onthe ground, measuring changes in the intensity of an optical signalbeing transmitted through the optical transmission line subjected to thevibration, and displacing the vibration generating means to determinethe location of the underground optical transmission line from adistribution of the intensity of the transmitted optical signal.
 2. Amethod of detecting an optical transmission line according to claim 1,wherein said underground optical transmission line is buried integrallywith another underground utility installation.
 3. A method of detectingan optical transmission line according to claim 1, wherein saidunderground optical transmission line is buried in parallel with anotherunderground utility installation.
 4. A method of detecting an opticaltransmission line according to claim 1, wherein said opticaltransmission line is detected by two-dimensionally locating said opticaltransmission line in terms of two different horizontal directions fromthe ground surface.
 5. A method of detecting an optical transmissionline according to claim 1, wherein said optical transmission line isdetected by vertically locating said optical transmission line from theground surface.
 6. A method of detecting an optical transmission lineaccording to claim 1, wherein a state of polarization of a signalapplied to the optical transmission line to be detected is modulated tochange the intensity of an output signal
 7. A method of detecting anoptical transmission line according to claim 1, wherein an opticalcoupling coefficient is modulated at the output terminal of the opticaltransmission line to change the intensity of an output signal of thetransmission line to be detected.
 8. A method of detecting an opticaltransmission line according to claim 1, wherein a single mode opticalfiber in the optical transmission line to be detected is used for thedetection.
 9. A method of detecting an optical transmission lineaccording to claim 1, wherein a multi-mode optical fiber in the opticaltransmission line is used for the detection and connected at a terminalthereof with a single mode optical fiber.
 10. A method of detecting anoptical transmission line according to claim 9, wherein a specklepattern appearing at the connection of the multi-mode optical fiber andthe single mode optical fiber is varied to change the intensity of theoutput signal of the multi-mode optical fiber of the opticaltransmission line.